Date converter, looking for good and easy way

Question

Hello,

I need to display a date in a different format. How can I achieve this easily? How do I go about it? Thanks for tips in advance.

The algorithm

Variant 1

//Month:  1=January, 2=February, 9=September, …, A=October, B=November
//Day:    standard from 01 to 31
//Year:   A=2017, B=2018, C=2019, … D=2020, E=2021, F=2022, G=2023, H=2024 , I= 2025
//Month:  A=January, B=February, …
//Day:    1=1st, 2=2nd, 9=9th, … 0=10th, A=11th, B=12th,…

• IB5
• IBA

---

Variant 2

//Year:   1=2017, … , 5=2021, 6=2022, ....9=2025, A=2026, B=2027
//Month:  1=January, 2=February, 9=September, …, A=October; B=November; C=December
//Day:    standard from 01 to 31

• 05.02.2025 --> 9205 (two digits for day) Year, Month, Day
• 05.02.2025 --> 925 (one or two digits for day)
• 11.02.2025 --> 9211 (one or two digits for day)

Answer 1

I would create a custom ICustomFormatter to allow you to convert the datetime values. Then you can use it with any of the standard formatting methods available in NET.

Here's a simple example that seems to work for your 2 specific cases. Note that it relies on C# switch expressions. If you're using an older version of C# then you'll need to replace that with something equivalent like if statements.

public class VariantCustomFormatter : IFormatProvider, ICustomFormatter
{
    public string Format ( string? format, object? arg, IFormatProvider? formatProvider )
    {
        if (arg is DateTime dt)
        {
            switch (String.IsNullOrEmpty(format) ? default(char) : format[0])
            {
                case 'V': return HandleVariant1(dt);
                case 'v': return HandleVariant2(dt);

                default: return HandleOtherFormats(format, arg);
            };
        };

        return HandleOtherFormats (format, arg);
    }

    public object? GetFormat ( Type? formatType )
    {
        if (formatType == typeof(ICustomFormatter))
            return this;

        return null;
    }

    private static string HandleVariant1 ( DateTime dt )
    {
        //YMD
        //Month = A + (Month - 1)
        //Day = 1-90A-...
        //Year = A + (Year - 2017)

        var yearCode = ((char)('A' + (dt.Year - 2017))).ToString();
        var monthCode = ((char)('A' + (dt.Month - 1))).ToString();

        //Using switch expression, if not supported then use IF
        var dayCode = dt.Day switch {
            < 10 => dt.Day.ToString(),
            > 10 => (dt.Day - 1).ToString("X"),
            _ => "0",
        };
        
        return yearCode + monthCode + dayCode;
    }

    private static string HandleVariant2 ( DateTime dt )
    {
        //YMDD
        //Month = 1-9A-C
        //Day = 1-31
        //Year = -2025 = 1..9, 2026+ = A+

        //Using switch expression, if not supported then use IF
        var yearCode = dt.Year switch {
            > 2025 => ((char)('A' + (dt.Year - 2026))).ToString(),
            _ => (dt.Year - 2016).ToString(),
        };
        var monthCode = dt.Month switch {
            <= 9 => dt.Month.ToString(),
            _ => ((char)('A' + (dt.Month - 1))).ToString(),
        };
        var dayCode = dt.Day.ToString("00");

        return yearCode + monthCode + dayCode;
    }

    private static string HandleOtherFormats ( string format, object arg )
    {
        if (arg is IFormattable)
            return ((IFormattable)arg).ToString(format, CultureInfo.CurrentCulture);

        return arg?.ToString() ?? "";
    }
}

This code is not optimized. You'll need to make it production ready. Usage example.

var items = new [] {
    DateTime.Parse("02/05/2025"),
    DateTime.Parse("02/11/2025"),    
};

foreach (var item in items)
{
    var formatProvider = new VariantCustomFormatter();

    Console.WriteLine(String.Format(formatProvider, "{0:V}", item));
    Console.WriteLine(String.Format(formatProvider, "{0:v}", item));
}